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Overview

The LEFT JOINreturns all matching records from the left table combined with the right table. Even if there are no matching records in the right table, the LEFT JOIN will still return a row in the result, but with NULL in each column from the right table.
LEFT JOIN is also known as LEFT OUTER JOIN.

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Syntax

We support table aliasing used in the LEFT JOIN clause.

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a) Basic Syntax

SELECT column_1, column_2...
FROM table_1
LEFT JOIN table_2
ON table_1.matching_field = table2.matching_field;
In the above syntax:
  1. SELECT column_1, column_2... defines the columns from both tables where we want the data to be selected.
  2. FROM table_1 defines the left table as the main table in the FORM clause.
  3. RIGHT JOIN table_2 defines the right table as the table the main table joins.
  4. ON table_1.matching_field = table2.matching_field sets the join condition after the ON keyword with the matching field between the two tables.

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b) Syntax with an Alias

You can use an alias to refer to the table’s name. The results will stay the same. It only helps to write the query easier. 
SELECT A.column_1, B.column_2...
FROM table_1 A //table_1 as A
LEFT JOIN table_2 B //table_2 as B
ON A.matching_field = B.matching_field;

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Example

item table 
CREATE TABLE item (
  item_no int NOT NULL,
  item_name text
);

INSERT INTO item
    (item_no,item_name)
VALUES
    (111,'Butter'),
    (113,'Tea'),
    (116,'Bread'),
    (119,'Coffee');

SELECT * FROM item;
It will create a table as shown below: 
+-----------+----------------+
| item_no   | item_name      |
+-----------+----------------+
| 111       | Butter         |
| 113       | Tea            |
| 116       | Bread          |
| 119       | Coffee         |
+-----------+----------------+
invoice table 
CREATE TABLE invoice (
  inv_no int NOT NULL,
  item int,
  sold_qty int,
  sold_price int
);

INSERT INTO invoice
    (inv_no, item, sold_qty, sold_price)
VALUES
    (020219,111,3,9000),
    (020220,116,6,30000),
    (020221,116,2,10000),
    (020222,116,1,5000),
    (020223,119,5,20000),
    (020224,119,4,16000);

SELECT * FROM invoice;
It will create a table as shown below: 
+----------+---------+-----------+-------------+
| inv_no   | item    | sold_qty  | sold_price  |
+----------+---------+-----------+-------------+
| 20219    | 111     | 3         | 9000        |
| 20220    | 116     | 6         | 30000       |
| 20221    | 116     | 2         | 10000       |
| 20222    | 116     | 1         | 5000        |
| 20223    | 119     | 5         | 20000       |
| 20224    | 119     | 4         | 16000       |
+----------+---------+-----------+-------------+
1) Based on the above tables, we can write a LEFT JOIN query as follows: 
SELECT item_no, item_name, sold_qty, sold_price
FROM item
LEFT JOIN invoice
ON item.item_no = invoice.item;
  • The item = left table, and the invoice = right table.
  • Then it combines the values from the item table using the item_no and matches the records using the item column of each row from the invoice table.
  • If the records are equal, a new row will be created with item_no, item_name, and sold_qty, sold_price columns as defined in the SELECT clause.
  • ELSE it will create a new row with a NULL value from the right table (invoice).
2) The above query will give the following result: 
+-----------+-------------+------------+---------------+
| item_no   | item_name   | sold_qty   | sold_price    |
+-----------+-------------+------------+---------------+
| 111       | Butter      | 3          | 9000          |
| 113       | Tea         | null       | null          |
| 116       | Bread       | 6          | 30000         |
| 116       | Bread       | 2          | 10000         |
| 116       | Bread       | 1          | 5000          |
| 119       | Coffee      | 5          | 20000         |
| 119       | Coffee      | 4          | 16000         |
+-----------+-------------+------------+---------------+
Based on the data from the item and invoice tables:
  • The result matches the total item stored in the item table: 4 items.
  • The result will display all the item’s data from the left table (item table), even if there is 1 item that hasn’t been sold.
  • The item id: 111 matches the item butter and has been sold for 3pcs/9000.
  • The item id: 113 matches the item tea but has never been sold. Thus the sold_qty & sold_price columns are filled with: null.
  • The item id: 116 matches the item Bread and has been sold three times, for 6pcs/3000, 2pcs/10000, and 1pc/5000.
  • The item id: 119 matches the item Coffee and has been sold two times, for 5pcs/20000 and 4pcs/16000.
An item can have zero or many invoices. An invoice belongs to zero or one item.
The following Venn diagram illustrates the LEFT JOIN: